1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device having a variable cell resistance Rcell in which a rate of charge discharge between two electrodes of the variable cell resistance Rcell differs according to the logical value of stored information.
2. Description of the Related Art
A nonvolatile memory device in which a precharge voltage is applied to a bit line, and a difference in rate of discharge of the precharge voltage is read out is known.
A nonvolatile semiconductor memory device to which such a readout method can be applied is typified by a (flash) EEPRPM.
Meanwhile, a resistance change type memory device is drawing attention as a nonvolatile memory device in which data is rewritable at high speed, to replace an FG type (flash) EEPROM.
As a resistance change type memory device, a so-called ReRAM is known in which a resistance change when conductive ions are input to or output from a conductive film within a variable cell resistance Rcell is made to correspond to a state of memory (see K. Aratani, etc., “A Novel Resistance Memory with High Scalability and Nanosecond Switching,” Technical Digest IEDM 2007, pp. 783-786, for example).
A system that performs verification readout at times of writing and erasure as in ordinary flash memories and the like is under consideration for a purpose of ensuring reliability of the rewriting characteristics, retention characteristics and the like of the ReRAM and for application of the ReRAM to multivalued memories (see Japanese Patent Laid-Open Nos. 2002-260377, 2005-510005, and 2004-164766 (hereinafter referred to as Patent Documents 1 to 3), for example).
Current control at a time of verification readout in an ordinary flash memory makes a readout current (sense current) substantially constant, and thus different threshold values are verified by changing the gate potential of a memory transistor. This operating system has an advantage in that sense timing, a load on a sense node, and the like are hardly dependent on the threshold values to be verified because of the constant operating current.